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Abstract:

A negative active material for a rechargeable lithium battery including a
flake powder including a plurality of flakes, each flake including a
plurality of silicon atoms and a plurality of oxygen atoms, wherein an
oxygen atom amount for each flake ranges from 5 wt % to 38 wt % based on
a total amount of silicon atoms and oxygen atoms, each flake having a
thickness ranging from 30 nm to 500 nm and a ratio of an average longest
dimension to the thickness ranging from 10 to 100.

Claims:

1. A negative active material comprising a flake powder that comprises a
plurality of flakes, each flake including: a plurality of silicon atoms
and a plurality of oxygen atoms, wherein an oxygen atom amount ranges
from about 5 wt % to about 38 wt % based on a total amount of silicon
atoms and oxygen atoms; a thickness ranging from about 30 nm to about 500
nm; and a ratio of an average longest dimension to a thickness ranging
from about 10 to about 100.

2. The negative active material of claim 1, wherein the average longest
dimension of each flake of the negative active material ranges from about
1 μm to about 20 μm.

3. A rechargeable lithium battery, comprising: a negative electrode
comprising a negative active material comprising a flake powder
comprising a plurality of flakes, each flake comprising a plurality of
silicon atoms and a plurality of oxygen atoms, wherein oxygen atom amount
ranges from about 5 wt % to about 38 wt % based on a total amount of
silicon atoms and oxygen atoms, each flake having a thickness ranging
from about 30 nm to about 500 nm and a ratio of an average longest
dimension to a thickness ranging from about 10 to about 100; a positive
electrode comprising a positive active material; and an electrolyte.

4. The rechargeable lithium battery of claim 3, wherein the average
longest dimension of each flake of the negative active material ranges
from about 1 μm to about 20 μm.

5. The rechargeable lithium battery of claim 3, wherein the negative
electrode further comprises: a binder; and a conductive material.

7. The rechargeable lithium battery of claim 5, wherein the binder
comprises an organic compound having an imide bond.

Description:

CLAIM OF PRIORITY

[0001] This application makes reference to, incorporates the same herein,
and claims all benefits accruing under 35 U.S.C. §119 from an
application for NEGATIVE ACTIVE MATERIAL FOR RECHARGEABLE LITHIUM BATTERY
AND RECHARGEABLE LITHIUM BATTERY INCLUDING THE SAME earlier filed in the
Japanese Patent Office on 23 Feb. 2010 and Korean Intellectual Property
Office on 22 Jul. 2010 and there duly assigned Japanese Patent
Application No. 2010-037422 and Korean Patent Application No.
10-2010-0071084

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This disclosure relates to a negative active material for a
rechargeable lithium battery, and a rechargeable lithium battery
including the same.

[0004] 2. Description of the Related Art

[0005] In recent times, due to reductions in size and weight of portable
electronic equipment, there has been a need to develop batteries for use
in the portable electronic equipment where the batteries have both high
performance and large capacity. Batteries generate electric power by
using materials capable of electrochemical reactions at positive and
negative electrodes.

[0006] For example, a rechargeable lithium battery generates electricity
due to change of chemical potentials when lithium ions are
intercalated/deintercalated at positive and negative electrodes. The
rechargeable lithium battery includes a material that can reversibly
intercalate/deintercalate lithium ions as positive and negative active
materials, and also an organic electrolyte or a polymer electrolyte.

[0007] As for negative active materials for a rechargeable lithium
battery, various carbon-based materials such as artificial graphite,
natural graphite, and hard carbon have been used because the carbon-based
materials have good cycle-life characteristics and safety
characteristics.

[0008] In order to provide a high-capacity rechargeable battery, there
have been efforts to improve utility of a carbon-based active material
and charge density per electrode volume. Recently, the rechargeable
lithium battery using a carbon-based material as a negative active
material exhibiting a capacity corresponding to a theoretical capacity
(372 mAh/g) of graphite have been developed. Furthermore, charge density
improvement also reaches a limit.

[0009] Therefore it is difficult to additionally improve the capacity of a
battery using currently available carbon-based materials. In order to
provide a high capacity rechargeable lithium battery, a metal material
including silicon (Si), tin (Sn), or the like having a higher charge and
discharge capacity compared to graphite has drawn attention as a negative
active material for a rechargeable lithium battery, which is disclosed in
Japanese Patent No. 2997741 and U.S. Pat. No. 5,395,711 to Tahara et al.

[0010] However, the volume of the metal-based material is too easily
changed during charging and discharging, so, in the rechargeable lithium
battery including the same, the negative active material layer is easily
collapsed and the physical and electrical bond can not be maintained in
the negative electrode. What is therefore needed is an improved negative
active material for a rechargeable lithium battery that improves capacity
but does not expand and contract upon charging and discharging.

SUMMARY OF THE INVENTION

[0011] One aspect of this disclosure provides a negative active material
for a rechargeable lithium battery having low volume changes during
charging and discharging, and having an excellent charge and discharge
cycle characteristics.

[0012] Another aspect of this disclosure provides a rechargeable lithium
battery including the negative active material for a rechargeable lithium
battery.

[0013] According to one aspect of the present invention, there is provided
a negative active material that includes a flake powder that includes a
plurality of flakes, each flake including a plurality of silicon atoms
and a plurality of oxygen atoms, wherein an oxygen atom amount ranges
from about 5 wt % to about 38 wt % based on a total amount of silicon
atoms and oxygen atoms, a thickness ranging from about 30 nm to about 500
nm and a ratio of an average longest dimension to a thickness ranging
from about 10 to about 100. The average longest dimension of each flake
of the negative active material may range from about 1 μm to about 20
μm.

[0014] According to another aspect of the present invention, there is
provided a rechargeable lithium battery that includes a negative
electrode comprising a negative active material comprising a flake powder
comprising a plurality of flakes, each flake comprising a plurality of
silicon atoms and a plurality of oxygen atoms, wherein oxygen atom amount
ranges from about 5 wt % to about 38 wt % based on a total amount of
silicon atoms and oxygen atoms, each flake having a thickness ranging
from about 30 nm to about 500 nm and a ratio of an average longest
dimension to a thickness ranging from about 10 to about 100, a positive
electrode comprising a positive active material and an electrolyte. The
average longest dimension of each flake of the negative active material
may range from about 1 μm to about 20 μm. The negative electrode
may also include a binder and a conductive material. The conductive
material may include a carbon-based material. The binder may include an
organic compound having an imide bond.

BRIEF DESCRIPTION OF THE DRAWING

[0015] FIG. 1 is a schematic view of a rechargeable lithium battery
according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Exemplary embodiments of the present invention will hereinafter be
described in detail. However, these embodiments are only exemplary, and
the present invention is not limited thereto.

[0017] The negative active material for a rechargeable lithium battery
according to one embodiment includes a flake powder including silicon
atoms and oxygen atoms, wherein the flake powder includes a plurality of
flakes, each flake has an oxygen atom amount ranging from about 5 wt % to
about 38 wt % based on a total amount of silicon and oxygen atoms, a
thickness ranging from about 30 nm to about 500 nm, and a ratio of
average longest dimension to thickness ranging from about 10 to about
100. Aspect ratio is, in general, the ratio between the longest and
shortest dimensions of a particle and is defined more specifically in
this instance as the ratio of the longest and shortest particle radii
that pass through the geometric center of the particle. The oxygen amount
is based on the total amount of silicon atoms and oxygen atoms (100 wt %)
included in the negative active material for a rechargeable lithium
battery.

[0018] The flake refers to one having a long plate shape having a longest
dimension of approximate 10 or more times the thickness, which may be
confirmed using an electron microscope or a particle size analyzer.

[0019] The plates of the negative active material for a rechargeable
lithium battery are aligned in parallel in a negative electrode, so it
may uniformly disperse pressure and distortion on the negative electrode.
As a result, the flakes may effectively compensate effects from volume
change during charging and discharging.

[0020] In the material including silicon atoms and oxygen atoms, the
oxygen atoms are present in a non-equilibrium state, for example, it may
include a state in which silicon particles are dispersed in an amorphous
matrix composed of silicon atoms and oxygen atoms.

[0021] The material including the silicon atoms and oxygen atoms may be
represented by SiOx (0.1≦x≦1) and the SiOx may
contain impurities together with silicon atoms and oxygen atoms in a
certain range as long as it does not deteriorate characteristics.

[0022] The SiOx has high capacity and excellent cycle-life
characteristics compared to a carbon material such as graphite and the
like. SiOx has a smaller volume change rate during charging and
discharging, and SiOx has excellent charge and discharge cycle
characteristics and less volumetric change compared to a particle
consisting of only silicon (Si).

[0023] In addition, as the SiOx has a flake shape having the same
size and shape as mentioned, most of the flakes are aligned in a
direction parallel to the current collector while fabricating the
negative electrode. As a result, the pressure and distortion due to the
volume change during charging and discharging are uniformly dispersed, so
the physical and electrical bonds are maintained in the negative
electrode after repeated charge and discharge cycles.

[0024] The SiOx is a flake powder having a thickness of about 30 nm
to about 500 nm and a ratio of an average longest dimension to a
thickness of about 10 to about 100.

[0025] The negative active material for a rechargeable lithium battery may
effectively compensate the effects of volume change during charging and
discharging since the flakes are aligned in parallel to each other in a
negative electrode, and pressure and distortion are uniformly dispersed
in the negative electrode.

[0026] For example, the negative active material may have an average major
diameter of about 1 μm to about 20 μm. If the average major
diameter ranges from about 1 μm to about 20 μm, the flakes may have
less affect on volume change due to expansion and shrinkage during
charging and discharging, and the flakes may not deteriorate the charge
and discharge capacity. Furthermore, the flakes may prevent cracks of the
active material powder that may be caused by the volume change during
charging and discharging and the flakes may improve the charge and
discharge cycle characteristics.

[0027] The SiOx may have an oxygen amount ranging from about 5 wt %
to about 38 wt %, or for example, from about 10 wt % to about 30 wt %, or
of about from 15 wt % to about 25 wt %. When the SiOx has an oxygen
amount within these ranges, they may maintain a high electron
conductivity to provide excellent charge and discharge capacity, and the
flakes may provide good charge and discharge cycle characteristics. On
the other hand, the oxygen amount of SiOx may be measured by inert
gas fused infrared absorption or the like. The oxygen amount refers to an
amount of oxygen based on the total amount of SiOx (100 wt %).

[0028] The flake SiOx may be obtained in accordance with the
following method. The flakes may be provided by using a raw material of
only silicon metal or a mixture of silicon metal and silicon monoxide
(SiO) and/or silicon dioxide (SiO2), forming an oxide thereof on the
surface of a substrate to an appropriate thickness by sputtering or
vacuum deposition under an atmosphere by controlling a partial pressure
of oxygen, separating the obtained layer from the substrate and
pulverizing the same.

[0029] The substrate may include a resin film such as polyethylene
telephthalate, polyethylene, polypropylene, or the like, and a metal foil
such as copper, stainless steel, or the like. The pulverizing of the
obtained layer may include wet-pulverizing in an organic solvent using a
ball mill.

[0030] According to another embodiment, a rechargeable lithium battery is
provided that includes the negative electrode including a negative active
material, a positive electrode including a positive active material, and
an electrolyte.

[0031] Rechargeable lithium batteries may be classified as lithium ion
batteries, lithium ion polymer batteries, and lithium polymer batteries
according to the presence of a separator and the kind of electrolyte used
therein. The rechargeable lithium batteries may have a variety of shapes
and sizes, and include cylindrical, prismatic, coin, or pouch-type
batteries, and may be thin film batteries or may be rather bulky in size.
Structures and fabricating methods for lithium ion batteries pertaining
to the present invention are well known in the art.

[0032] Turning now to FIG. 1, FIG. 1 is a schematic view of a rechargeable
lithium battery according to one embodiment of the present invention. As
shown in FIG. 1, the rechargeable lithium battery 1 includes a negative
electrode 2, a positive electrode 4, and a separator 3 interposed between
the negative electrode 2 and the positive electrode 4, an electrolyte
(not shown) impregnated in the negative electrode 2, the positive
electrode 4, and the separator 3, and a sealing member 6 sealing a
battery case 5. Such a rechargeable lithium battery 1 is fabricated by
sequentially stacking the negative electrode 2, positive electrode 4, and
separator 3, spiral-winding the resultant, and accommodating the
spiral-wound body into the battery case 5.

[0033] The negative electrode 2 includes a current collector and a
negative active material layer disposed thereon. The negative active
material layer includes a negative active material, a binder, or
optionally a conductive material.

[0034] The current collector may be selected from the group consisting of
a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a
nickel foam, a copper foam, a polymer substrate coated with a conductive
metal, and combinations thereof. Alternatively, the current collector may
be omitted by providing a negative electrode where the negative active
material is pressed and compressed.

[0035] The negative active material may include the above-described
negative active material for a rechargeable lithium battery, and
furthermore, at least one of a material that reversibly
intercalates/deintercalates lithium ions, a lithium metal, a lithium
metal alloy, a material being capable of doping lithium, or a transition
metal oxide that may be further used in combination with the negative
active material. The negative active material layer may further include
additives such as a filler, a dispersing agent, or the like, as needed.

[0036] The material that reversibly intercalates/deintercalates lithium
cations is a carbon material, and any carbon-based negative active
material generally used in a lithium cation rechargeable battery may be
used, such as crystalline carbon, amorphous carbon, or a combination
thereof. Non-limiting examples of the crystalline carbon include
graphite, such as shapeless, sheet-type, flake-type, spherical, or
fibrous natural graphite or artificial graphite. Examples of the
amorphous carbon include soft carbon or hard carbon, mesophase pitch
carbide, or fired coke.

[0038] Examples of the material being capable of doping lithium include
Si, a Si-A alloy (where A is an element selected from the group
consisting of an alkali metal, an alkaline-earth metal, a group 13
element, a group 14 element, a transition element, a rare earth element,
and combinations thereof, and is not Si), Sn, SnO2, a Sn-G alloy
(where G is an element selected from the group consisting of an alkali
metal, an alkaline-earth metal, a group 13 element, a group 14 element, a
transition element, a rare earth element, and combinations thereof, and
is not Sn), or mixtures thereof. At least one of these materials may be
mixed with SiO2. The elements A and G may be selected from the group
consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db,
Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag,
Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, or a
combination thereof. Examples of the transition metal oxide include
vanadium oxide, lithium vanadium oxide, or the like.

[0039] The binder improves binding properties of negative active material
particles with one another and with a current collector. Examples of the
binder may include a compound including an imide bond, polyvinyl alcohol,
carboxylmethyl cellulose, hydroxypropyl cellulose, polyvinylchloride,
carboxylated polyvinylchloride, polyvinylfluoride, an ethylene
oxide-containing polymer, polyvinylpyrrolidone, polyurethane,
polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,
polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene
rubber, an epoxy resin, or the like. The compound including an imide bond
may be polyimide, polyamideimide, polybenzimidazole, or the like. An
exemplary of the binder may be a compound including an imide bond such as
polyimide, polyamideimide, polybenzimidazole, or the like.

[0040] The conductive material is included to improve electrode
conductivity. Any electrically conductive material may be used as a
conductive material unless it causes a chemical change. Examples of the
conductive material include natural graphite, artificial graphite, carbon
black, acetylene black, ketjen black, a carbon fiber, a metal powder or a
metal fiber including copper, nickel, aluminum, silver, and so on, a
polyphenylene derivative, or mixtures thereof.

[0041] The positive electrode 4 includes a positive active material layer
and a current collector supporting the positive active material layer.
The current collector may be aluminum (Al), but is not limited thereto.
The positive active material layer includes a positive active material.
The positive active material may include a compound that may
intercalate/deintercalate lithium ions such as a lithiated intercalation
compound. The positive active material may include a composite oxide
including at least one selected from the group consisting of cobalt,
manganese, and nickel, as well as lithium.

[0043] In the above Chemical Formulae, A is selected from the group
consisting of Ni, Co, Mn, and a combination thereof; B is selected from
the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth
element, and a combination thereof; D is selected from the group
consisting of O, F, S, P, and a combination thereof; E is selected from
the group consisting of Co, Mn, and a combination thereof; F is selected
from the group consisting of F, S, P, and a combination thereof; G is
selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V,
and a combination thereof; Q is selected from the group consisting of Ti,
Mo, Mn, and a combination thereof; X is selected from the group
consisting of Cr, V, Fe, Sc, Y, and a combination thereof; and J is
selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and a
combination thereof.

[0044] The positive active material may include the compound with a
coating layer, or a mixture of the compound without the coating layer and
the compound coated with the coating layer. The coating layer may include
at least one coating element compound selected from the group consisting
of an oxide of the coating element, a hydroxide of the coating element,
an oxyhydroxide of the coating element, an oxycarbonate of the coating
element, and a hydroxycarbonate of the coating element. The compound for
the coating layer may be either amorphous or crystalline. The coating
element included in the coating layer may be selected from the group
consisting of Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr,
and combinations thereof. The coating process may include any
conventional processes as long as it does not cause any side effects on
the properties of the positive active material (e.g., spray coating,
immersing), which are well known to persons having ordinary skill in this
art, so a detailed description thereof is omitted.

[0045] The positive active material layer may include the binder that
improves binding properties of the positive active material particles to
each other and to a current collector. Examples of the binder include at
least one selected from the group consisting of a polyimide, polyvinyl
alcohol, carboxylmethyl cellulose, hydroxypropyl cellulose, diacetyl
cellulose, polyvinylchloride, carboxylated polyvinylchloride,
polyvinylfluoride, an ethylene oxide-containing polymer,
polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,
polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene
rubber, acrylated styrene-butadiene rubber, an epoxy resin, nylon, or the
like, but is not limited thereto.

[0046] The positive active material layer may include a conductive
material that improves electrical conductivity of the positive electrode.
Any electrically conductive material may be used as a conductive agent
unless it causes a chemical change. Examples of the conductive material
may include carbon-based materials such as natural graphite, artificial
graphite, carbon black, acetylene black, ketjen black, carbon fiber, or
the like; metal-based materials including a metal powder or a metal fiber
of copper, nickel, aluminum, or the like; conductive polymer materials
such as polyphenylene derivatives; or mixtures thereof.

[0047] The positive active material layer may be included with a suitable
amount of additives such as a filler, a dispersing agent, and the like,
as needed.

[0048] The negative electrode 2 and the positive electrode 4 may be
fabricated by a method including mixing an active material, a binder, or
the like in a solvent to produce an active material composition, and
coating the composition on a current collector. Any electrically
conductive material can be used as a conductive agent unless it causes a
chemical change. The solvent includes N-methylpyrrolidone and the like,
but is not limited thereto.

[0049] The electrolyte includes a non-aqueous organic solvent and a
lithium salt. The non-aqueous organic solvent serves as a medium for
transmitting ions taking part in the electrochemical reaction of a
battery. The non-aqueous organic solvent serves as a medium for
transmitting ions taking part in the electrochemical reaction of a
battery. Examples of the carbonate-based solvent may include dimethyl
carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC),
methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl
carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC),
butylene carbonate (BC), or the like. Examples of the ester-based solvent
may include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl
acetate, methyl propionate, ethyl propionate, butyrolactone, decanolide,
valerolactone, mevalonolactone, caprolactone, or the like. Examples of
the ether-based solvent may include dibutyl ether, tetraglyme, diglyme,
1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane,
2-methyltetrahydrofuran, tetrahydrofuran, or the like. Examples of the
ketone-based solvent may include cyclohexanone or the like. Examples of
the alcohol-based solvent may include ethanol, isopropyl alcohol, or the
like, and examples of the aprotic solvent include nitriles such as R--CN
(wherein R is a C2 to C20 linear, branched, or cyclic hydrocarbon, a
double bond, an aromatic ring, or an ether bond), amides such as
dimethylformamide, dioxolanes such as 1,3-dioxolane, sulfolanes, or the
like.

[0050] The non-aqueous organic solvent may be used singularly or in a
mixture. When the organic solvent is used in a mixture, the mixture ratio
can be controlled in accordance with desirable battery performance.

[0051] The carbonate-based solvent may include a mixture of a cyclic
carbonate and a linear carbonate. The cyclic carbonate and the chain
carbonate are mixed together in the volume ratio of 1:about 1 to 1:about
9, and when the mixture is used as an electrolyte, the electrolyte
performance may be enhanced.

[0052] In addition, the electrolyte of the present invention may further
include mixtures of carbonate-based solvents and aromatic
hydrocarbon-based solvents. The carbonate-based solvents and the aromatic
hydrocarbon-based solvents are preferably mixed together in the volume
ratio of about 1:1 to about 30:1.

[0053] The aromatic hydrocarbon-based organic solvent may be represented
by the following Chemical Formula 1.

##STR00001##

[0054] In Chemical Formula 1, R1 to R6 are independently
hydrogen, a halogen, a C1 to C10 alkyl, a C1 to C10 haloalkyl, or
combinations thereof.

[0056] The non-aqueous electrolyte may further include vinylene carbonate
or an ethylene carbonate-based compound of the following Chemical Formula
2.

##STR00002##

[0057] In Chemical Formula 2, R7 and R8 are independently
hydrogen, a halogen, a cyano group (CN), a nitro group (NO2), and a
C1 to C5 fluoroalkyl group, provided that at least one of R7 and
R8 is a halogen, a nitro group (NO2), or a C1 to C5 fluoroalkyl
group and R7 and R8 are not simultaneously hydrogen.

[0058] The ethylene carbonate-based compound includes difluoroethylene
carbonate, chloroethylene carbonate, dichloroethylene carbonate,
bromoethylene carbonate, dibromoethylene carbonate, nitroethylene
carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. The use
amount of the additive for improving cycle life may be adjusted within an
appropriate range.

[0059] The lithium salt supplies lithium ions in the battery, operates a
basic operation of a rechargeable lithium battery and improves lithium
ion transport between positive and negative electrodes. Non-limiting
examples of the lithium salt include at least one supporting salt
selected from LiPF6, LiBF4, LiSbF6, LiAsF6,
LiN(SO2C2F5)2, Li(CF3SO2)2N,
LiN(SO3C2F5)2, LiC4F9SO3, LiClO4,
LiAlO2, LiAlCl4,
LiN(CxF2x+1SO2)(CyF2y+1SO2), (where x and y are
natural numbers), LiCl, LiI, and LiB(C2O4)2 (lithium
bis(oxalato) borate, LiBOB). The lithium salt may be used at about a 0.1
to about 2.0M concentration. When the lithium salt is included at the
above concentration range, electrolyte performance and lithium ion
mobility may be enhanced due to optimal electrolyte conductivity and
viscosity.

[0060] The rechargeable lithium battery may further include a separator
between a negative electrode and a positive electrode, as needed.
Non-limiting examples of suitable separator materials include
polyethylene, polypropylene, polyvinylidene fluoride, and multi-layers
thereof such as a polyethylene/polypropylene double-layered separator, a
polyethylene/polypropylene/polyethylene triple-layered separator, and a
polypropylene/polyethylene/polypropylene triple-layered separator.

[0061] Hereinafter, the embodiments are illustrated in more detail with
reference to examples. These examples, however, should not in any sense
be interpreted as limiting the scope of the present invention.
Furthermore, what is not described in this specification can be
sufficiently understood by those who have knowledge in this field and
will not be illustrated here.

Preparation of Negative Active Material

Example 1

[0062] A substrate is a 100 μm-thick polyethylene terephthalate film,
and it is coated with a SiOx (x=0.14) film on its surface in a
thickness of 200 nm according to the following RF sputtering.

[0063] The target is a metal silicon having a purity of 6N, and the
distance of between the substrate and the target is set into 65 mm; the
sputter gas is an argon-0.1% oxygen mixed gas; and the chamber internal
pressure is controlled to 0.1 Pa during the reaction. The output of high
frequency is controlled to 400 W.

[0064] The provided SiOx (x=0.14) is separated from the substrate and
is wet-pulverized for 30 minutes by a ball mill while using an ethanol
solvent and is then dried to provide an active material A.

[0065] The obtained active material A has a flake shape. The active
material A is measured for longest dimension using electron microscope
photography, and it shows that the average is 10 μm which is
distributed from 5 μm to 20 μm, there is no powder broken out in a
thickness direction, and all powder maintains the thickness of coating
process which is 200 nm. As a result, the ratio of an average longest
dimension to thickness is 50.

[0066] The oxygen amount included in the active material A is 5.0 wt % if
measured using inert gas fused infrared absorbance. The oxygen amount
refers to the oxygen (O) atoms based on the total amount of SiOx
(x=0.14) (100 wt %) included in the active material A.

Example 2

[0067] An active material B having a flaky shape is prepared in accordance
with the same procedure as in Example 1, except that it is wet-pulverized
for 5 hours in an ethanol solvent using a ball mill instead of
wet-pulverizing for 30 minutes.

[0068] The active material B has an average longest dimension of 4 μm,
distributed from 2 μm to 8 μm, there is no powder broken in a
thickness direction, and all powder maintains the thickness of the
coating process which is 200 nm. As a result, a ratio of average longest
dimension to thickness is 20.

[0069] The oxygen amount included in the active material B is 5.8 wt %.
The oxygen amount refers to the oxygen (O) atoms based on the total
amount of SiOx (x=0.16) (100 wt %) included in the active material
B.

Example 3

[0070] An active material C having a flaky shape is prepared in accordance
with the same procedure as in Example 1, except that an argon-1.0% oxygen
mixed gas is used as the sputter gas instead of the argon-0.1% oxygen
mixed gas.

[0071] The active material C has the same shape as the active material A
of Example 1, and the oxygen amount is 9.8 wt %. The oxygen amount refers
to the oxygen (O) atoms based on the total amount of SiOx (x=0.27)
(100 wt %) included in the active material C.

Example 4

[0072] An active material D having a flaky shape is prepared in accordance
with the same procedure as in Example 1, except that the sputter gas is
an argon-3.0% oxygen mixed gas instead of the argon-0.1% oxygen mixed
gas.

[0073] The active material D has the same shape as the active material A
of Example 1, and the oxygen amount is 20.4 wt %. The oxygen amount
refers to the oxygen (O) atoms based on the total amount of SiOx
(x=0.56) (100 wt %) included in the active material D.

Example 5

[0074] An active material E having a flaky shape is prepared in accordance
with the same procedure as in Example 1, except that the sputter gas is
an argon-5.0% oxygen mixed gas instead of the argon-0.1% oxygen mixed
gas.

[0075] The active material E has the same shape as the active material A
of Example 1, and the oxygen amount is 29.1 wt %. The oxygen amount
refers to the oxygen (O) atoms based on the total amount of SiOx
(x=0.8) (100 wt %) included in the active material E.

Comparative Example 1

[0076] An active material F is prepared in accordance with the same
procedure as in Example 1, except that the SiOx film is formed to a
thickness of 5 μm according to RF sputtering instead of 200 nm.

[0077] The active material F has a shape of block (short sheet) and a
thickness of 1 μm to 3 μm and an average longest dimension of 5
μm distributed from 2 μm to 10 μm, and a ratio of average
longest dimension to thickness of 1.7 to 5.

[0078] The oxygen amount included in active material F is 5.0 wt %. The
oxygen amount refers to the oxygen (O) atoms based on the total amount of
SiOx (x=0.14) (100 wt %) included in the active material F.

Comparative Example 2

[0079] An active material G is prepared in accordance with the same
procedure as in Example 1, except that it is dry-pulverized in a mortar
instead of wet-pulverizing in a solvent of ethanol for 30 minutes using a
ball mill.

[0080] The active material G has an average major diameter of 50 μm
distributed from 35 μm to 100 μm. There is no powder broken in the
thickness direction, and all powder maintains the thickness of 200 nm
which is the thickness on forming the layer, so a ratio of average major
diameter to thickness is 250.

[0081] The oxygen amount included in active material G is 5.0 wt %. The
oxygen amount refers to the oxygen (O) atoms based on the total amount of
SiOx (x=0.14) (100 wt %) included in the active material G.

Comparative Example 3

[0082] An active material H is prepared in accordance with the same
procedure as in Example 1, except that the sputter gas includes an
argon-7.0% oxygen mixed gas instead of the argon-0.1% oxygen mixed gas.

[0083] The active material H has the same shape as the active material A
of Example 1, and the oxygen amount is 39.1 wt %. The oxygen amount
refers to the oxygen (O) atoms based on the total amount of SiOx
(x=1.08) (100 wt %) included in the active material H.

[0084] (Fabrication of Rechargeable Lithium Battery)

[0085] Each negative active material obtained from Examples 1 to 5 and
Comparative Examples 1 to 3 is mixed with a binder of polyamideimide and
a conductive material of denka black (manufactured by Denki kagaku koyo
Co. Ltd.) in a weight ratio of 80:10:10 and dispersed in
N-methyl-2-pyrrolidone (NMP) to provide a negative active material
slurry. Each negative active material slurry is coated on 10 μm-thick
copper foil (negative electrode current collector) at 5 mg/cm2,
dried at 130° C. and thermo-set at 250° C., and punched in
a disc having a diameter of 13 mm and pressed to provide a negative
electrode.

[0086] Using the electrode and a counter electrode of metal lithium, a
coin cell is fabricated. A separator for the coin cell is a polyethylene
porous layer having a thickness of 20 μm, and the electrolyte is
prepared by mixing a lithium salt of 1.2M LiPF6 into a solvent of
ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of
3:7.

[0087] (Assessment of Cycle Characteristics)

[0088] Each coin cell obtained from Examples 1 to 5 and Comparative
Examples 1 to 3 is charged at a constant current of 0.05 C to 0.005V
(Li/Li.sup.+) and discharged at 0.05 C to a final voltage of 1.4V
(Li/Li.sup.+), and is then measured for discharge capacity at the first
cycle. It is charged at a constant current of 0.5 C to 0.005V
(Li/Li.sup.+) and discharged at 0.5 C to 1.4V (Li/Li.sup.+), and this is
repeated for 50 cycles. The results are shown in the following Table 1.

[0089] In the coin cells according to Examples 1 to 5, it is confirmed
that the discharge capacity at the first cycle decreases upon increasing
the oxygen amount included in SiOx. On the other hand, the 51st
cycle/2nd cycle discharge capacity ratio showing the discharge capacity
retention during charging and discharging cycles is improved by this
increased oxygen amount to provide improved battery characteristics.

[0090] On the other hand, in Comparative Example 1, since the active
material does not have a flake shape, it is significantly affected from
the volume change due to the expansion and shrinkage, so the discharge
capacity is very low after 51 cycles.

[0091] In Comparative Example 2, since the ratio of average longest
dimension to thickness is excessive even though it has a flake shape, it
is affected by the volume change during charging and discharging to cause
a significant crack, and thereby the electrolyte is decomposed on the
surface exposed by the crack to reduce the charge and discharge cycle
characteristics.

[0092] In addition, in Comparative Example 3, since the oxygen amount of
SiO, is excessive, the electrical conductivity reduced, and the charge
and discharge capacity is significantly lowered.

[0093] While this disclosure has been described in connection with what is
presently considered to be practical exemplary embodiments, it is to be
understood that the invention is not limited to the disclosed
embodiments, but, on the contrary, is intended to cover various
modifications and equivalent arrangements included within the spirit and
scope of the appended claims. Therefore, the aforementioned embodiments
should be understood to be exemplary but not limiting in any way.